Electrostatic transfer method and apparatus for coating articles with a fluidized composition



Apnl 26, 1966 J. c. BARFORD ETAL 3,248,253

ELECTROSTATIC TRANSFER METHOD AND APPARATUS FOR COATING ARTICLES WITH A FLUIDIZED COMPOSITION Filed June 15, 1963 6 Sheets-Sheet 1 'IIII'I'IIIII Fig.1

Fig-2 April 26, 1966 J. c. BARFORD ETAL 3,248,253 ELECTROSTATIC TRANSFER METHOD AND APPARATUS FOR COATING ARTICLES WITH A FLUIDIZED COMPOSITION 6 Sheets-Sheet 2 Filed June 15, 1963 Fig- 3 ELECTROSTATIC TRANSFER METHOD AND APPARATUS FOR COATING ARTICLES WITH A FLUIDIZED COMPOSITION Filed June i3, 1965 6 Sheets-Sheet 5 i' 2 If 3 fw. Jr-;,,ff;":; i i; 5 "E I Fig .4 9

2 5 5; 23 22 IL.- Lz-i 2b 2 24 :3 3

Fig.5 Fig.6

April 1966 J. c. BARFORD ETAL 3,248,253

ELECTROSTATIC TRANSFER METHOD AND APPARATUS FOR COATING ARTICLES WITH A FLUIDIZED COMPOSITION Filed June 13, 1963 6 Sheets-Sheet 4 Fig.7

p 1966 J. c. BARFORD ETAL 3,248,253

ELECTROSTATIC TRANSFER METHOD AND APPARATUS FOR COATING ARTICLES WITH A FLUIDIZED COMPOSITION Filed June 13, 1963 6 Sheets-Sheet 5 p 25, 1966 J. c. BARFORD ETAL 3,248,253

ELECTROSTATIC TRANSFER METHOD AND APPARATUS FOR COATING ARTICLES WITH A FLUIDIZED COMPOSITION Filed June 15, 1963 6 Sheets-Sheet 6 United States Patent ELECTRGSTATIC TRANSFER METHOD AND AP- PARATUS FOR COATING ARTICLES WITH A FLUIDIZED COMPOSITION John C. Barford, Stainton, Rotherham, England, and

Marcel A. R. Point, Grenoble, and Guy Nicolas, Meylan, France, assignors to Sames, Societe Anonyme dc Machines Electrostatiques, Grenoble, France, a French joint-stock company Filed June 13, 1963, Ser. No. 287,554

Claims priority, application Great Britain, June 14, 1962, 22,944/62; France, July 31, 1962, 905,604, Patent 1,338,913; Aug. 3, 1962, 905,975, Patent 1,338,453; Feb. 20, 1963, 925,370, Patent 83,092

27 Claims. (Cl. 117-17) The present invention relates to a coating method and apparatus and more particularly to a method and apparatus for coating various articles through the agency of a bath of fluidized coating powder.

According to coating methods and apparatus resorted to hitherto and operating through the agency of a bath of fluidized coating powder, the powder customarily is in the form of finely divided solid fusible particles which are suspended in air or other suitable fluid. The article to be coated is first heated to a comparatively high temperature and is then immersed in the bath. The heat from the article serves to at least partly melt the powder in contact therewith, thus ensuring the adherence of the powder to the article. The article is then wtihdrawn from the bath, and a second heating treatment is applied which provides the final melting of the powder. In some cases, the article is thereafter subjected to a smoothing procedure to improve the surface characteristics of the coating.

The prior methods and apparatus employed to form coatings of this type have exhibited certain disadvantages. For example, the need for applying high temperature to the article to be coated prior to the coating operation has often proved time consuming and costly. In addition, the requirement that the article be physically immersed in the bath frequently has necessitated the design of special conveying equipment or has otherwise proved disadvantageous. Furthermore, and this has been of special moment for articles having comparatively intricate shapes, it heretofore has been diificult to coat the various surfaces of the article uniformly and evenly without excessive wastage of the coating material.

One general object of this invention, therefore, is to provide a novel and economical method and apparatus for coating various articles through the agency of a bath of fluidized coating powder. I a

More specifically, it is an object of this invention to provide such a method and apparatus in which the preliminary heating of the articles prior to the coating operation in most cases is eliminated.

Another object of the invention is to provide a method and apparatus of the character indicated which, in certain good embodiments, avoids the necessity for physically immersing the articles in the fluidized powder.

Still another object of the invention is to provide a continuous process for applying a smooth and uniform coating to the articles in a rapid and straightforward manner.

A further object of the invention is to provide a novel and economical apparatus for coating various articles which is economical to manufacture and thoroughly reliable in operation.

The article coating method according to one illustrative embodiment of the invention comprises electrostatically charging by electric charging means a bath of fluidized powder with a potential different from that of the article 3,248,253 Patented Apr. 26, 1966 to be coated, such that the charged powder particles are attracted by and secured as a uniform layer over one or more surfaces of the article. A thermic treatment is then applied to transform the powder layer into a continuous coating film.

Experience shows that, by reason of the electric charge acquired by the particles of powder, the particles adhere perfectly to the article to be coated, so that a smooth and uniform coating layer is obtained.

According to certain particularly advantageous embodiments of the method of the invention, the article or articles to be coated are brought into cooperating but spaced relationship with the bath of fluidized particles. The potential difference between the particles and the bath draws the particles out of the bath so as to impinge onto the surfaces of the articles. This arrangement enables the application of coating layers of widely varying thicknesses to the article surfaces without the necessity for immersing the articles in the bath.

According to another embodiment, the articles to be coated are caused to enter the fluidized bath, thereby enabling the application of coating layers of even greater thicknesses to the articles.

According to a further embodiment of the method, the bath and associated charging means are shifted with respect to the article during the coating operation. As a result, even comparatively large article surfaces are uniformly coated in a rapid and straightforward manner.

The present invention also relates to arrangements for the execution of the method described herein. Thus, in certain good embodiments, there is provided a vat which is supplied with air or other fluidizing gas and includes a porous partition adapted to carry the bath of powder. A series of pointed members is supported above the partition and is supplied with an electrical charging potential, thus electrostatically charging the bath. Conveyor means is provided for carrying each article to be coated into electrostatically cooperating relationship with the bath. The potential difference between the article and the bath draws powder out of the bath and onto one or more surfaces of the article to form a layer of powder thereon. The article is then advanced through a kiln or other suitable heating means to transform it into a film.

According to another embodiment of the apparatus, the electrostatic charging means is carried by a support member having a surface of a configuration which is in substantial conformity with the surface to be coated. With this arrangement, an exceedingly uniform coating is applied to the article.

According to still another embodiment, a plurality of vats containing the fluidized powder are spaced apart at intervals throughout the desired field of projection of powder onto the article, thereby further facilitating the coating of comparatively large articles.

The foregoing and other objects and advantages of the invention will appear more clearly and fully from a reading of the following description, given by way of example, of various preferred embodiments, reference being made to the accompanying drawings, wherein:

FIGURE 1 is a diagrammatic view, with certain parts shown in section, of one embodiment of a coating apparatus for the execution of the improved method in accordance with the invention;

FIGURES 2 through 5 are diagrammatic views in general similar to FIGURE 1 but illustrating four further embodiments of coating apparatus adapted to execute said method;

FIGURE 6 is a fragmentary cross-section through line VIVI of FIGURE 5;

FIGURE 7 is a transverse vertical sectional view, partially diagrammatic, of another embodiment of an apparatus for executing the method according to the invention;

FIGURES 8a and 8b are longitudinal vertical sectional views of two modifications of portions of the apparatus illustrated in FIGURE 7, with certain parts shown broken away;

FIGURE 9 is an exploded perspective view of still another embodiment of an apparatus in accordance with the invention, with certain parts omitted for purposes of clarity;

FIGURE 10 is a transverse vertical sectional view of the apparatus illustrated in FIGURE 9, together with certain additional parts and with the article to be coated shown in a different position;

FIGURE 11 is a diagrammatic view, partially in section, of a further embodiment of an apparatus according to the invention; and

FIGURE 12 is a fragmentary vertical sectional view of a still further modification of a portion of an apparatus in accordance with the invention.

Turning to FIGURE 1, there is shown a vat 2, preferably of electrically insulating material, which is supported above the ground through the agency of insulating feet 8. Compressed air or other suitable fluid is introduced into the lower portion of the vat 2 through a conduit or channel 5. The air then passes through the pores of a porous partition 4 which is horizontally mounted within the vat 2 at a level immediately above the infeed end of the channel 5. Said partition 4 may be made of metal or other conductive material or of insulating material, such as ceramic ware for instance. In the example illustrated in FIGURE 1, said partition is made of metal and rigidly supports electrostatic charging means 10. The charging means 10 comprises a series of charging portions in the form of upstanding pointed members of generally conical configuration. In the FIGURE 1 embodiment, these members are of equal length and lie wholly within a bath 3 of fluidized powder particleswhich are carried inside the vat 2 above the partition and serve to define an upper bath surface 50. The members are connected through the partition to the negative terminal of a high DC. voltage generator 1, the other terminal of which is grounded.

The electrostatic charging means 10 are made preferably of a semi-conductive material having a high electrical resistivity so as to provide a protection in a conventional manner whenever a spark is produced as a consequence of a fortuitous contacting between the charging means and any part which may be grounded or otherwise at a potential sufiicient to produce arcing. If the charging means are made of an electrically good conductive material, they may be connected with the negative terminal of the generator 1 through a protecting resistance 51 to reduce the energy evolved in the case of the fortuitous jumping of a spark.

The articles 7 to be coated are at room temperature and are positioned or caused to move horizontally above the bath 3 by a conveyor 9, from which the articles are suspended in a conventional manner. The conveyor 9 is grounded so that the articles are likewise at ground potential. Because of the potential difference between the negatively charged powder in the bath 3 and the articles 7, the articles electrostatically attract individual powder particles, as shown schematically by the reference character 6 in FIGURE 1. The particles drawn out of the bath and onto the articles form a layer of powder on the articles which is held in place by the electrostatic attraction therebetween. As the conveyor 9 continues its movement and carries the thus coated articles past the vat 2, the articles are advanced through suitable heating means (not shown in FIGURE 1) which serves to fuse the layer of powder on each article to transform it into a permanent film coating the article.

Although a wide variety of coating powders may be employed in the bath 3, particularly good results are obtained in cases in which the powder has comparatively good insulation properties. In some embodiments, the volume resistivity of the powder preferably is greater than about 100,000 ohm-centimeters. With this arrangement, the possibility of substantial current flow between the powder and an article being coated is maintained at a minimum, and the electrostatic adhesion between the article and the powder serves to afiirmatively maintain the powder on the coated surfaces as the article moves from above the vat 2 through the subsequent heating operation.

Representative powders that have exhibited particular utility as coating materials include a wide variety of thermoplastic resins, such as polyethylene, polypropylene, nylon, vinyl chloride, the cellulosics, acrylics, etc., various thermosetting resins, e.g., the epoxies, several phenolic type resins and many of the silicones, for example, and various inorganic powders. Examples of suitable powders of this latter class include talc, various vitreous materials, metallic oxides and phosphors.

Of course, it will be understood that the foregoing coating materials are but illustrative, and numerous other materials may be employed without departing from the spirit or scope of the invention as defined by the appended claims.

The size of the individual powder particles in the coating bath 3 may vary over a relatively wide range. For a given material, smoother and more uniform coatings are generally obtained through the use of comparatively fine particles of uniform size. In cases in which exceedingly smooth finish characteristics are desired, the size of the individual particles preferably is below mesh U.S. Standard for best results.

In accordance with several good embodiments, the articles to be coated are of relatively conductive material, when compared with the conductivity of the powder particles in the bath 3. Suitable. article materials in accordance with these embodiments include not only materials which are conventionally recognized as having good conductive properties, but also other materials, such as paper and wood, for example, which under normal humidity conditions are at least mildly conductive. In general, an extremely wide variety of articles may be readily coated in accordance with the invention.

The coating applied to the various articles is highly uniform even on articles having irregular or complex shapes. The individual negatively charged particles of powder are attracted by the grounded articles and tend to seek out thinly coated or uncovered areas. For a single passage of a given article over the vat 2, the thickness of the coating frequently ranges up to 20 mils or more, depending in part upon the articles rate of movement. In cases in which even greater coating thicknesses are desired, the article is moved over the vat a sufficient number of times to provide the proper thickness.

Referring to FIGURE 2, there is shown an apparatus which is particularly advantageous in the coating of articles in the form of metal strips, wires, grids, etc., according to mass production methods. The article 14 to be coated is continuously fed at a uniform rate off a grounded drum 13 and is wound after its treatment over a takeup drum 12, the-drive of the drums being ensured through any suitable conventional means (not shown). The article passes over a vat 2a which is generally similar to the vat 2 (FIGURE 1) described above and which contains the electrostatically charged powder bath 3 having the upper surface 50. As it moves over the bath, the article is spaced at a suitable height above the surface level thereof. After the article has been coated with powder, it passes for instance through a kiln 11 inside which the powder melts so as to form a film coating the article. The article is then wound over the drum 12.

In the embodiments of FIGURES 1 and 2, the articles are providedwith a smooth and uniform layer of powder as they are carried over the bath without the necessity for preheating the articles prior to coating. As a result, the overall cost of the coating operation is substantially reduced and its efiiciency increased. In other embodiments, particularly in cases in which an especially thick coating is desired, the articles may be subjected to a preliminary heating step prior to the application of the powder. FIGURE 3 is illustrative of one of these latter embodiments which is of particular utility in the coating of articles in the form of broad metal strips, for example. Two powder distributing vats 2a and 2a are then provided, each vat including a bath similar to that illustrated in FIGURE 2 for coating a corresponding surface of a metal strip 15. The strip 15 passes first through a heating kiln 16 and then horizontally over the lower powder distributing vat 2a in cooperating relationship with the upper surface 50 of the bath therein, whereby the lower surface of the strip is coated with powder. The strip 15 is then wound over the grounded roller 17, the uncoated surface of the strip being in contact with the roller. Upon leaving the roller 17, the strip passes above the upper surface 50 of the bath in the upper powder distributing vat 2a where the surface of the strip which has not yet been coated receives its layer of powder, as explained hereinabove. This being done, the strip which is coated over both surfaces returns to the kiln 16 inside which the powder melts and forms the final coating on both sides of the strip. a

The strip 15 is raised to a predetermined temperature by its initial passage through the kiln 16, so as to produce a complete or partial melting of the powder as it contacts the strip. Such preheating is of particular utility in cases in which an especially thick coating is to be provided. In addition, the preheating of the strip serves to further increase the adhesion between the outer surface of the strip and the powder particles as the strip is advanced from the first vat 2a around the drum 17 toward the second vat 2a.

In all cases, it is possible to associate with the vat carrying the bath suitable means (not shown) for feeding it with powder, so as to provide the maintenance of a substantially constant level of powder inside said vat.

Turning now to FIGURE 4, the ionized bath illustrated therein is substantially similar to that illustrated precedingly and includes the upper surface 50. The articles 18 to be coated are grounded by the conveyor 19 carrying them, which conveyor assumes above the vat 2 carrying the bath a downwardly incurved shape as shown at 1%, so that the articles 18 enter the fluidized and electrostatically charged powder and are immersed therein,

whereby they are covered electrostatically with the amount of powder required for their coating. As the conveyor 19 continues its movement, the articles 18 are withdrawn from the fluidized bath and pass through a kiln 20, inside which the powder melts and forms the desired coat. In some cases, the articles are advanced through a preliminary heating kiln 21 prior to entering the bath.

Referring to FIGURES 5 and 6,'there is shown a vat 2b which is in general similar to the vat 2a (FIGURE 2) but which includes a stufling box 26 forming an aperture in the vat wall at the input end thereof a short distance above the metal partition 4. The partition 4 fixedly supports an annular carrier member 23 of conductive material having a series of electrostatically charging means 10a protruding at spaced intervals from the inner cylindrical surface thereof. The charging means 10a are of generally conical configuration and are completely immersed beneath the upper surface of the powder bath 3. A high negative potential is supplied to the means 10a either directly from the generator 1 and the member 23 or through the agency of the partition 4. The means 10a are of different lengths and are given different angular settings with respect to the member 23 so that the electrostatic charging tips are positioned at intervals around a wire 22 to be coated, as best shown in FIGURE 6.

The wire 22 is led along its feed path at a uniform 6 rate from a grounded supply spool 24 through a pre liminary heating kiln 25 and the stufling box 26 to the vat 2b. As the wire 22 passes through the electrostatically charged bath 3, the potential difference between the wire and the bath draws powder 6 out of the bath and onto the wire to form a layer of powder thereon. After leaving the vat 2b, the wire moves through a kiln 27 which is spaced from the vat adjacent the outfeed side thereof. The kiln 27 serves to melt the powder layer and from the coat for the wire. The wire then passes through air and/ or water-cooled means 28 to reduce its temperature before being wound over a take-up or storing spool 29.

The arrangement is such that a uniform and virtually pinhole free coating is applied over the entire peripheral surface of the wire.

The embodiment of FIGURE 7 includes, to either side of a vertical plane in which a conveyor 31 is adapted to move in a generally horizontal direction, two insulating supports 32 and 33 carried by corresponding insulating feet 34 and 35. Each support 32, 33 carries three superposed vats 36, 37, 38 or 36', 37', 38' which are fed with fluidizing air through corresponding pipes 39, 40, 41 or 39', 40', 41' shunted off a conduit 42, 42' through which compressed air flows. The vats 36, 37, 38 and 36', 37', 38' are in the form of elongated channels which extend horizontally and are spaced apart one above the other on the facing sides of the associated supports 32 and 33. Each vat includes a porous horizontal partition 43 which lies between the air intake .and a series of electrostatic charging means 44, 44' thereabove. The electrostatic charging means 44, 44' are connected through cables 45 with the negative terminal of a high DC. voltage supply 46, the other terminal of which is grounded, and are generally similar to the various charging means described heretofore. However, as best shown at 44a and 44b in FIGURES 8a and 8b, the charging means are spaced above the corresponding partition43 and are carried by longitudinally extending conductive rods 45a. Each of these rods is suitably connected to one of the conductors 45.

The articles 48 to be coated are suspended from the conveyor 31 through electrically conductive hooks 47. The conveyor is connected to ground to thus maintain the articles at ground potential. Compressed air is continuously fed through the conduits 42'and 42' to fluidize the powder particles in the vats 36, 37, 38 and 36, 37', 3S, and the electrostatic charging means 44 and 44' are supplied with voltage from the generator 46. As the articles 48 move at a uniform rate between the panelshaped supports 32 and 33, powder from the vats is electrostatically deposited on the article surfaces, the powder being replenished through suitable hoppers (not shown) to maintain the level of the bath in each vat substantially constant. Upon the application of the powder coating to each article, the article is heated in a manner similar to that previously described to transform the powder into a permanent film.

In certain embodiments of the invention, the individual electrostatic charging means, such as the electrostatic charging means 44a, shown in FIGURE 8a, for example, projects slightly above the surface level of the fluidized bath. This arrangement is particularly advantageous for certain coating powders, e.g., vinyl chloride. In other satisfactory embodiments, the charging means are en-' tirely immersed in the fluidized bath, as shown by the charging means 44b in FIGURE 8b.

The apparatus of FIGURES 9 and 10 includes an insulating pedestal 60 provided with a projecting section 61 on which are mounted three powder-carrying vats 62, 63, 64. The vats 62 and 63 are of annular configuration and extend horizontally around the section 61 to form a closed circuit. The vat 64 is horizontally supported on the upper portion of the section 61 and is generally similar to the various vats shown in FIGURE 7.

Each of the vats 62, 63, 64 is supplied with fluidizing gas and includes electrostatic charging means supplied with a high DC. potential as described above.

The article 65 to be coated is positioned in cooperating relationship with the projecting section 61, and the upper surface of the section 61 is in substantial conformity with the facing surface of the article. As a result, a substantially closed space is formed between the article and the vats 62, 63, 64.

The article 65, which illustratively comprises a bath tub, is suspended by electrically conductive hangers 66 secured to a grounded support 67. The coating operation is an intermittent one, and the admission of compressed air-and the energization of the charging means take place only during a predetermined period, after the article is in its proper (FIGURE position. Powder from the vats 62, 63, 64 is deposited on the adjacent article surface in a uniform layer. Upon the removal of the article from the coating apparatus, heat is applied to fuse this layer into a permanent film.

According to the embodiment illustrated in FIGURE 11, there is provided an elongated channel-shaped vat 80 of insulating material which serves as a carrier for the bath of fluidized powder. A flexible conduit 81 feeds compressed air to the vat, while a flexible conductor 82 supplies a high DC. voltage to the electrostatic charging means 83. The vat 80 is supported at the outer end of a horizontal carrier arm 84 which extends in a direction transverse to that of the vat. The arm 84 is rigidly affixed to a carriage 85 adapted to move along vertical rails 86 under the control of a chain system 87, for example. The chain system 87 produces a reciprocatory movement of the carriage, the amplitude of which movement is adjustable by changing the positions of two abutments 88 and 89 defining the ends of the travel of the carriage.

The articles 90 to be coated are suspended by conductive hooks 91 from a conveyor 92 which is grounded. The conveyor 92 advances the articles in a horizontal direction in parallelism with the longitudinal direction of the vat 80 and in spaced, cooperating relationship therewith. As the articles move past the vat, the vat assumes a vertical reciprocatory movement in the direction of the arrows F and F. Individual particles of powder illustrated at I are electrostatically projected from the vat and sweep throughout the entire adjacent surface 93 of the article 90 to provide a uniform and complete layer of particles thereon. The particles from the vat 80 also are deposited over portions of the upper and lower surfaces of the articles as the vat reciprocates.

Of course, if the article is to be coated on both sides, it may either move in registry with a second vat (not shown in FIGURE 11) arranged adjacent the opposite surface of the article, or it may again be passed in front of the vat 80 after being rotated one-half revolution about its vertical axis. Upon the application of powder to the various article surfaces, heat is applied in the manner described above to form a film coating the article.

In some embodiments, the vat 80 is automatically fed with powder at one end of its reciprocatory stroke, preferably the upper end, through the agency of a suitable hopper 52. The termination of the upward movement of the vat may open a sluice 53 for the hopper to permit a predetermined amount of powder to be discharged therefrom. i

In certain arrangements, the movable arm 84 may carry a number of vats, rather than the single vat 80 shown in FIGURE 11.

FIGURE 12 is illustrative of a powder-carrying vat 93 which may be employed in a manner similar to any of the various vats described heretofore. The vat 93 is of insulating material and is provided with a porous partition 94 of conductive or semi-conductive material which is connected at 95 with the high voltage supply. The par- 8 tition 94 supports a fluidized bath 96 of reduced thickness. serves as electrostatic charging means and is effective to charge the powder particles in the bath with a potential different from that of the article to be coated.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

1. A method for coating an article by means of a bath of fluidized powder, comprising electrically charging the bath of fluidized powder with a potential different from that of an article to be coated, positioning the article and the bath in spaced-apart electrostatically cooperating relationship with each other, such that the potential difference between said fluidized powder and said article draws powder out of the bath and onto the article to form a layer of powder thereon, and fusing said layer to transform it into a smooth and uniform film coating the article.

2. A method for coating an article by means of a bath of fluidized powder, comprising electrically charging the bath of fluidized powder with a potential different from that of an article to be coated, moving the article to be coated into spaced-apart, electrostatically cooperating relationship with said bath, maintaining said article at substantially room temperature while in said cooperating relationship, the potential difference between said fluidized powder and said article drawing powder out of the bath onto the article to form a layer of powder on the latter, and applying heat to said layer to transform it into a smooth and uniform film coating the article.

3. A method for coating an article by means of solid powder particles, comprising electrically charging a fluidized bath of the powder particles with a potential different from that of an article to be coated, moving the article and the bath of particles into spaced-apart electrostatically co-operating relationship with each other, maintaining said article at substantially room temperature while in said cooperating relationship, the potential difference between said particles and said article drawing particles out of said bath and onto the article to form a layer of particles thereon during the time said article and said bath are spaced-apart, and thereafter applying heat to said layer to transform it into a smooth and uniform film coating the article.

4. A method for coating an article, comprising form- .ing a bath of fluidized powder, electrically charging the bath of fluidized powder with a potential different from that of an article to be coated, said electrically charged fluidized bath having an observable free upper surface, and positioning the article in spaced-apart electrostatically, cooperating relationship with the free upper surface of said bath, such that the potential difference between said fluidized powder and said article draws powder out of the bath and onto the article to form a layer of powder thereon.

5. A method for coating an article by means of a bath of fluidized powder, comprising electrically charging a plurality of charging members with a potential different from that of an article to be coated, forming a bath of fluidized powder around said charging members so as to submerge the same wholly within said bath, to thereby charge said powder with said potential, positioning the article and said bath in spaced-apart electrostatically cooperating relationship with each other, such that the potential difference between said fluidized powder and said article draws the powder onto the article to form a layer of powder thereon, and thereafter fusing said layer to,

transform it into a smooth and uniform film coating the article.

In the FIGURE 12 arrangement, the partition 6. A method for coating an article by means of solid powder particles, consisting of applying an electrical charging-potential to a plurality of charging members, forming a bath of fluidized powder around said charging members so as to submerge the same wholly within said bath, to thereby charge said powder with said potential, maintaining an article having at least mildly conductive properties at a potential different from that of the fluidized powder particles, positioning the article in spacedapart, electrostatically cooperating relationship with said bath, the potential difference between said particles and said article drawing particles onto the article to form a layer of particles thereon, and fusing said layer to transform it into a smooth and uniform film coating the article.

7. A method for coating an article consisting of forming a bath of fluidized powder, applying an electrical potential to the bath of fluidized powder, said electrically charged fluidized bath havingan observable free upper surface, maintaining an article to be coated at a potential different from that of the potential applied to said bath, moving said article into spaced apart, electrostatically cooperating relationship with the free upper surface of said bath, the potential difference between said fluidized powder and said article drawing powder out of the bath onto the article to form a layer of powder on the latter, said article being at substantially room temperature while in said cooperating relationship, and applying heat to said layer to transform it into a smooth and uniform film coating the article.

8. A method for coating an article by means of a bath of fluidized powder, comprising electrically charging the bath of fluidized powder with a potential different from that of the article to be coated, applying preliminary heat to said article, positioning said article in spaced-apart, electrostatically cooperating relationship with said bath, the

potential difference between said fluidized powder and said article drawing powder out of the bath onto the article, and thereafter applying additional heat to said article to transform the powder thereon into a film coating the article.

9. A method for coating an article by means of fluidized powder, comprising electrically charging a plurality of baths of fluidized powder with a potential different from that of an article to be coated, advancing the article past said baths in spaced-apart, electrostatically cooperating relationship therewith, the potential difference between said fluidized powder and said article drawing powder out of said baths and onto the article to form a layer of powder thereon, and fusing said layer to transform it into a smooth and uniform film coating the article.

10. A method for coating an article my means of a bath of fluidized powder, comprising electrically charging the bath of fluidized powder with a potential diiferent from that of an article to be coated, positioning the article to be coated and the bath in spaced-apart, electrostatically cooperating relationship with each other such that the potential difference between said fluidized powder and said article draws powder out of the bath onto the, article to form a layer of powder on the latter, and shifting said bath with respect to said article while said bath and said article are in said spaced-apart relationship.

11. A method for coating articles by means of solid powder particles, comprising applying fluid to said particles to form a bath of fluidized powder, electrically charging the bath with a potential different from that of successive articles to be coated, each of said articles having at least mildly conductive properties, applying preliminary heat to said articles, continuously advancing said articles along a feed path and into spaced-apart, electrostatically cooperating relationship with said bath, the potential difference between said fluidized powder and said articles drawing powder out of the bath onto the articles, and thereafter applying additional heat to said articles as they move along their path to transform the powder thereon into a smooth and uniform film coating each article.

12. Apparatus for coating an article, comprising means for forming a bath of fluidized powder, charging means for electrically charging the bath of fluidized pow-der with a potential different from that of an article to be coated, said electrically charged fluidized bath having an observable free upper surface, and means for positioning the article in spaced-apart, electrostatically cooperating relationship with the free upper surface of said bath, such that the potential difference between said fluidized powder and said article draws powder out of the bath and onto the article to form a layer of powder thereon.

13. Apparatus for coating an article by means of a bath of fluidized powder, comprising a vat adapted to carry a bath of said powder, charging means carried within said Vat for electrically charging said bath with a potential different from that of an article to be coated, said charging means being wholly submerged within the bath of powder, means for positioning said article in spaced-apart, electrostatically cooperating relationship with said bath, the potential difference between said fluidized powder and said article drawing powder out of the bath and onto the article to form a layer of powder thereon, said article being at substantially room temperature during the formation of said layer, means for heating said layer to transform it into a smooth and uniform film coating the article, and means for carrying the coated article to the heating means.

14. Apparatus for coating an article by means of solid powder particles, comprising a vat adapted to carry said particles, means for feed-ing fluid to said vat to form a bath of fluidized powder, charging means supported within said vat for electrically charging said bath with a potential diflerent from that of an article to be coated, and means for positioning said article in spaced-apart, electrostatically cooperating relationship with said bath, the potential difference between said fluidized powder and said article drawing powder out of the bath and onto the article to form a layer of powder thereon.

15. Apparatus for coating an article by means of a bath of fluidized powder, comprising a vat including a porous partition adapted to carry a bath of fluidized powder, means for feeding a fluidizing gas through said porous partition, powder charging means adjacent said porous partition for electrically charging said bath with a potential different from that of an article to be coated, and means for carrying said article into spaced-apart, cooperating electrostatically relationship with said bath, the potential difference between said fluidized powder and said article drawing powder out of the bath and onto the article to form a layer of powder there-on during the time said article and said bath are in said spaced-apart, cooperating relationship.

16. Apparatus of the character set forth in claim 15, in which said powder charging means comprises at least one pointed member of semi-conductive material.

17. Apparatus for coating an article by means of a bath of fluidized powder, comprising a vat including a porous horizontal partition adapted to carry a bath of fluidized powder, means for feeding fluid upwardly through said porous partition, powder charging means carried adjacent said porous partition for electrically charging said bath with a potential d'iflferent from that of an article to be coated, said charging means including an upstanding pointed member which protrudes above the surface level of said bath, and means for carrying said article into spaced-apart, electrostatically cooperating relationship with said bath, the potential difference between said fluidized powder and said article drawing the powder out of the bath and onto the article .to form a layer of powder thereon.

18. Apparatus for coating an article by means of a bath of fluidized powder, comprising a support member located at a small distance from an article to be coated, said member including a surface thereon in substantial conformity with the facing surface of said article, a vat carried by said support member and including a porous partition adapted to carry a bath of fluidized powder, means for supporting said article in spaced-apart, electrostatically cooperating relationship with said bath of fluidized powder, means for fee-ding a fluidizing gas through said porous partition, and powder charging means carried adjacent said partition in spaced juxtaposition with said article for electrically charging said bath with a potential different from that of the article, said potential difference drawing powder out of the bath and onto the article to form a layer I of powder thereon.

19. Apparatus for coating an article by means of a bath of fluidized powder, comprising an elongated vat including a porous horizontal partition adapted to carry a bath of fluidized powder, means for feeding a fluidizing gas upwardly through said porous partition, powder charging means carried adjacent said partition for electrically charging said bath with a potential different from that of an article to be coated, conveyor means for carrying said article into spaced-apart, electrostatically cooperating relationship with said bath, the potential difference between said fluidized powder and said article drawing powder out of the bath and onto the article to form a layer of powder thereon, means for shifting the vat carrying said bath with respect to said article during the formation of said layer, and heating means for thereafter fusing said layer to transform it into a smooth and uniform film coating the article.

20. Apparatus for coating articles by means of a bath of fluidized powder, comprising an elongated vat including a porous horizontal partition adapted to carry a bath of fluidized powder, means for feeding a fluidizing gas upwardly through said porous partition, powder charging means including a series of upstanding pointed members carried by said partition for electrically charging said bath with a potential different from that of successive articles to be coated, each of said articles having at least mildly conductive properties, first and second heating means spaced from said bath, and conveyor means for continuously advancing said articles past said first heating means, then into spaced-apart electrostatically cooperating relationship with said bath and then past said second heating means, the potential difierence between said fluidized powder and said articles drawing powder out of the bath and onto the articles while in said cooperating relationship to form a layer of powder on each article, said second heating means thereafter fusing each said layer to transform it into a smooth and uniform film coating the corresponding article.

21. Apparatus for coating an article by means of a bath of fluidized powder, comprising a support member located at a small distance from an article to be coated, said support member having a surface thereon in substantial conformity with the facing surface of said article, vat means carried by said support member and including a porous partition adapted to carry a bath of fluidized powder, means for supporting said article in spaced-apart, electrostatically cooperating relationship with said bath of fluidized powder, means for feeding a fluidizing gas through said porous partition, a series of elongated charging members carried adjacent said partition and extending toward the surface of said article, and means for applying a high DC. potential to said charging members, to charge the particles of powder within said bath with a potential different from that of the article, said potential difference drawing powder out of the bath and onto the article to' form alayer of powder thereon during the time said bath and said article are in said spaced-apart, cooperating relationship.

p 22. Apparatus for coating an article by means of a bath of fluidizedpowder, comprising a support member of insulating material located at a small distance from an article to be coated, said support member having a surface thereon in substantial conformity with the facing surface of said article, a plurality of vats carried by said support member, each of said vats including a horizontal porous partition adapted to carry a bath of fluidized powder, means for feeding a fluidizing gas through the porous partitions, means for positioning said article in spacedapart, electrostatically cooperating relationship with each of the fluidized baths and for maintaining said article at ground potential, a series of upstanding pointed members of semi-conductive material carried wholly within each of said baths, means for applying a high negative D.C. potential to said pointed members, to charge the particles of powder within said baths with a corresponding potential, the potential difference between said article and said particles simultaneously drawing powder out of each of said baths and onto the article to form a layer of powder thereon during the time said baths and said article are in said spaced-apart, cooperating relationship, heating means for fusing said layer to transform it into a smooth and uniform film coating the article, and means for carrying the coated article to the heating means.

23. Apparatus for coating an article, comprising container means having a gas-pervious wall adapted to support a body of coating powder thereon, means for discharging fluidizing gas upwardly through said pervious wall and said body to maintain a fluidized bath of the pow der within said container means, said fluidized bath having an observable free upper surface, electrode means carried within said container means, means for apply-' ing a high electric potential to said electrode means to charge the individual powder particles to a corresponding potential, means for supporting an article to be coated in an operative position external to said fluidized bath and in spaced-apart, electrostatically cooperating relationship with the upper free surface thereof, and means electrically connecting the article to a potential different from said electrode potential, whereby said article when supported in said operative position creates an electrostatic field causing charged particles to rise from said free upper surface of the bath along lines of force of said field to coat the surface of the article.

24. Apparatus as defined in claim 23, wherein said container means comprises a plurality of individual containers each having a body of coating powder therein, said fluidizing gas being discharged through the powder in each of said containers, said electrode means including at least one electrostatic charging member associated with each of said containers.

25. Apparatus as defined in claim 23, wherein said article comprises a continuous elongated strip-like article, said supporting means being arranged to feed said striplike article along a pair of vertically-spaced, generally horizontal paths, said container means including a first container disposed beneath the under surface of said article as it moves along the lower of said paths and a second container disposed beneath the under surface of said article as it moves along the upper of said paths so as to coat both surfaces of said article.

26. Apparatus for coating the inner surface of a concave article, comprising a plurality of containers each adapted to support a body of coating powder therein, means for discharging fluidizing gas upwardly through the body of powder in each of said containers to maintain a fluidized bath of said powder in each container, each of the fluidized baths having an upper boundary zone, electrode means carried within each container, means for applying a high electric potential to said electrode means for charging the particles of powder to a corresponding potential, a support member supporting said plurality of containers in relative positions such as to define an over-all configuration generally corresponding to the configuration of said inner surface of the article, means for positioning said article in spaced-apart, electrostatically cooperating relationship with the upper boundary zones of the fluidized baths, and means for electrically connecting the article to a potential different from the potential of said particles, whereby the positioning of said article in said spaced relationship creates electrostatic fields having lines of force extending between each of said fluidized baths and a related portion of said inner article surface, to thereby cause charged particles to simultaneously rise from the boundary zone in each bath along said lines of force to coat said inner article surface.

27. A method for coating an article, comprising discharging a fiuidizing gas through a body of pulverulent coating substance to provide a fluidized bath of said substance, electrically charging the pulverulent substance in the bath to a high electric potential, said electrically charged fluidized bath having an observable free upper surface in a quiescent position during the discharge of said fluidizing gas therethrough, presenting an article to be coated into an operative position external to said fluidized bath and in spaced-apart, electrostatically cooperating relationship with said free upper surfiace, and electrically connecting said article to a potential different from said charging potential, whereby the presentation of said article to said operative position creates an electrostatic field causing charged particles to rise from said free upper surface of the bath along lines of force of said field and to settle over the surface of said article.

References Cited by the Examiner UNITED STATES PATENTS 2,152,077 3/1939 Meston et al 117-175 2,370,636 3/1945 Carlton 117-1|7 2,686,141 8/1954 Sawyer 117-17 2,844,489 7/1959 Gemmer 117-22 3,004,861 1,0/1961 Davis 11718 3,008,826 11/1961 Mott et al 11721 X 3,019,126 1/1962 Bartholomew 1l7 17 3,032,816 5/ 1962 Zimmerli.

WILLIAM D. MARTIN, Primary Examiner.

G. L. HUBBARD, Assistant Examiner. 

4. A METHOD FOR COATING AN ARTICLE, COMPRISING FORMING A BATH OF FLUIDIZED POWER, ELECTRICALLY CHARGING THE BATH OF FLUIDIZED POWER WITH A POTENTIAL DIFFERENT FROM THAT OF AN ARTICLE TO BE COATED, SAID ELECTRICALLY CHARGED FLUIDIZED BATH HAVING AN OBSERVABLE FREE UPPER SURFACE, AND POSITIONING THE ARTICLE, IN SPACED-APART ELECTROSTICALLY, COOPERATING RELATIONSHIP WITH THE FREE UPPER SURFACE OF SAID BATH, SUCH THAT THE POTENTIAL DIFFERENCE BETWEEN SAID FLUIDIZED POWER AND SAID ARTICLE DRAWS POWER OUT OF THE BATH AND ONTO THE ARTICLE TO FORM A LAYER OF POWDER THEREON.
 12. APPARATUS FOR COATING AN ARTICLE, COMPRISING MEANS FOR FORMING A BATH OF FLUIDIZED POWDER, CHARGING MEANS FOR ELECTRICALLY CHARGING THE BATH OF FLUIDIZED POWDER WITH A POTENTIAL DIFFERENT FROM THAT OF AN ARTICLE TO BE COATED, SAID ELECTRICALLY CHARGED FLUIDIZED BATH HAVING AN OBSERVABLE FREE UPPER SURFACE, AND MEANS FOR POSITIONING THE ARTICLE IN SPACED-APART, ELECTROSTICALY COOPERATING RELATIONSHIP WITH THE FREE UPPER SURFACE OF SAID BATH, SUCH THAT THE POTENTIAL DIFFERENCE BETWEEN SAID FLUIDIZED POWDER AND SAID ARTICLE DRAWS POWDER OUT OF THE BATH AND ONTO THE ARTICLE TO FORM A LAYER OF POWDER THEREON. 